Thermoelectric element and thermocouple



Patented Augi l, 1933 -UNlTE D STATES PATENT OFFICE THERMGCOUPLE Otto Hermann, St. Louis, Ma, assignor to Thermo Electric Company, St. Louialtilo" a Corporation oi Missc No Drawing. Application as so, real Serial No. 554,166

1y produced and a thermo-electric member espe-- cially adapted for use as the negative member of a theme-couple.

My invention consists principally in a thermoelectric member composed of an alloy of antimony, arsenic and zinc, antimony being the base metal. The invention further consists in the thermo-electric member and thermo-couple hereinafter described and claimed.

The negative thermo-electric member embodying my invention is an alloy composed of antimony, arsenic and zinc, the preferred range of ingredients being from to 63% antimony, 34 to 37% zinc and l to 4.5% arsenic. Typical compositions are: 60.7% antimony, 36.3% zinc, 3% arsenic; 62.7% antimony, 36.3% zinc, 1% arsenic.

Said alloy has been found to be highly negative against all commercial metals and alloys, including silver, alloys of silver and nickel and alloys of copper and nickel. A thermo-coupla having this alloy as the negative member and an alloy containing 55% copper and 45% nickel as the positive member produces an electro-motive force of .09 volt at. a difierence in temperature between the junctions as low as 300 C.

According to the preferred process of making said negative alloy metal, the antimony is melted and heated to a temperature from '70 to 100 above its melting point and the arsenic is added thereto. The arsenic is quickly dissolved in the melted antimony without any appreciable loss of the arsenic by evaporation. The zinc is then added to the arsenic-antimony mixture and readily alloys therewith.

The alloy is very hard and is strong enough tively large current. Since the ratio of the two The principal object of the.

conductivities, electrical and thermal, in both members (especially in the negative member) is not equal to unity, the electrical conductance in both members being smaller than the thermal conductance, the difierence in their sizes with respect to their cross sectional areas can not correspond simply to the ratio of their electrical conductance, but the cross sectional areas of the two elements must bear a quantitative relation to one another based on both the electrical and thermal conductance. other conditions being equal, the maximum electrical efilciency .of a thermocouple is obtained when the ratio of the cross sectional areas of the two members of the thermo-couple equals where s and 1 denote the two conductivities of one member and s and 1 those of the other member. It has been proved by experiment that when the sizes of the members with respect to their cross sectional areas are determined by the above ratio, the electrical conductance of the thermocouple and the flow of current in the same approach a maximum, while the heat carried on by conduction from the hot to the cold junction is at a minimum, The ratio of the cross sectional area of the positive element, when composed of 55% copper and 45% nickel, to the cross sectional area of the negative member is about 1 to 48 or more depending greatly upon the relative quantities of the component metallic elements contained in the negative alloy metal. The arsenic, in particular, is an important factor.

In view of the great differences between the physical properties of the alloy material and --those of the positive material, the process of uniting the members of the thermo-couple is of prime importance. According to my preferred process, the negative alloy material is cast around the end portion of the positive member, said positive member having the form of a wire or ribbon. If the positive member is composed of silver or of silver alloy, the negative material may be cast directly thereon. When the positive member is composed of other material, such as the hereinbefore mentioned alloy of 55% copper and 45% nickel, one or both ends of the positive thermo-couple member must be covered with 10 a film of silver, after which the molten negative alloy material may be cast around the positive member. It has been found by experiment that the negative alloy metal makes a firm and persistent contact with said layer of silver or with an alloy containing silver, Without the presence 05 silver in the positive member, or as a coating thereon, the negative member can not be made to make proper contact therewith and imperfect contact results in high contact resistance and reduction in the output afid ef ficiency o! the thermo-couple. Thermo-couples embodying the hereinbeiore described members Joined together by the hereinbeiore described process have very little contact resistance atthe Joints in addition to the inherent ohmic resistance of the two elements. The thermo-couples have a long and useful life which consists of an alloy of 65% copper and.

45% nickel and the negative element or an allay of 60-63% antimony, 34-37% zinc and 14.5% arsenic.

O'I'IO HERHANN. 

